Polyphenylene ether (PPE) resins are readily combinable with alkenyl aromatic resins in virtually all proportions and a great variety of thermoplastic products have been provided. These PPE-based compositions possess good chemical and physical properties such as impact strength, the ability to be flame retarded, good electrical properties as well as good processing properties. As a result, PPE compositions have found great utility in injection, foam and blow molding applications particularly in the automotive, appliance, industrial and electronic areas.
As noted, injection molding is an important process for fabricating PPE-based parts. One of the key processing properties of a thermoplastic resin is its melt behavior during the molding process. Improvements in melt behavior often correspond to great increases in processing efficiency and profitability. The ability of the resin to adequately fill the mold in ever shorter cycle times can be extremely advantageous to the thermoplastic molding industry and can result in lower costs to the end users of such thermoplastics.
It has now been discovered that low molecular weight copolymers of styrene and alpha methyl styrene are surprisingly effective for improving the melt flow behavior of PPE-alkenyl aromatic compositions without the attendant thermal instability normally associated with other low molecular weight styrenic resins.
Additionally it has been discovered that in a PPE-high impact polystyrene (HIPS) composition the partial replacement of HIPS by a combination of low molecular weight poly(styrene-co-alpha methyl styrene) (PS/PAMS) and a thermoplastic elastomer (such as a styrene-butadiene block copolymer) will not only achieve the expected improvement in impact strength, but will unexpectedly attain good or improved flow properties. This latter discovery is particularly important because it is often necessary or even critical in certain thermoplastic applications to maintain melt flow and heat distortion temperature (HDT) values above minimum specified values. It is not always feasible to achieve this effect by the use of plasticizers or by changing the PPE: alkenyl aromatic resin ratio.
Copolymers of styrene and alpha methyl styrene are particularly useful in compositions of the present invention because they possess a unique combination of chemical and physical properties which can be exploited to good advantage in such compositions. PS/PAMS copolymers have a higher HDT compared to polystyrene homopolymer. PS/PAMS copolymers have a higher de-polymerization temperature compared to poly alpha methyl styrene homopolymer.
Furthermore, PS/PAMS copolymers offer increased thermal stability compared to similar commercially available copolymers which additionally contain unsaturated aliphatic comonomers. Examples of these include PICCO 5140 resin and NEVEX 100 resin.
Additionally, PS/PAMS copolymers offer greater melt flow improvements compared to higher molecular weight styrenic homopolymers or copolymers.
The aforementioned advantages of PS/PAMS copolymers have been utilized in compositions of the present invention to provide polyphenylene ether based products which exhibit improved melt flow behavior even when such products are impact modified with rubbery additives or are flame retarded with halogenated compounds which tend to decrease the melt flow of the compositions.